N. R. Zahniser,*$ G. A. Gerhardt,*t$ A. F. Hoffman,* and C. R. Lupica*$ Departments of *Pharmacology, +Psychiatry, and *Neuroscience Program University of Colorado Health Sciences Center Denver, Colorado 80262 Voltage-Dependency of the Dopamine Transporter in Rat Brain The neuronal dopamine transporter (DAT) is critical for terminating DA neurotransmission. Uptake of DA by DAT is dependent on cotransport of Na' and C1P ions. Results of biochemical studies from several labs (1) suggest that two Na+ ions and one CIP ion are cotransported with each DA molecule. DA is also positively charged at physiological pH. Thus, at a minimum, there should be a net inward movement of two positive charges with each transport cycle, thereby generating an inward current. The suggestion that DAT is an electro- genic transporter also predicts that the translocation of DA, or DA uptake, should be voltage-dependent. A number of investigators ( 1) have previously shown that uptake of [3H]DA into rat striatal preparations is diminished by agents (e.g., elevated KCI concentrations, veratridine, batrachotoxin, and oua- bain) that induce membrane depolarization. However, in these preparations it is difficult to distinguish definitively the effects of agents such as these on membrane potential from their effects on the ionic gradients required for DA uptake. In cells such as Xenopus oocytes, membrane potential can be manipulated independently of ionic gradients. Using Xenopus oocytes expressing the cloned human (h) DAT and the two-electrode voltage clamp technique, Sonders and colleagues (2) recently demonstrated that hDAT is an electrogenic transporter. DA = 2.4 pM) induces an inward, transport-associated current at poten- tials below - 20 mV. The magnitude of these transport-associated currents is voltage-dependent. Furthermore, hDAT uptake velocity is also dependent on membrane potential. Under voltage clamp, membrane hyperpolarization in- creases DA uptake velocity in hDAT-expressing oocytes, whereas depolarization diminishes it (-25% change/3O mV). This latter result is interesting for several reasons. First, it suggests a novel mechanism by which neuronal activity can transiently regulate DAT velocity. Second, it suggests that membrane hyperpo- larization, by receptors such as presynaptic D2 DA autoreceptors, could also influence DAT activity. Indeed, several groups have demonstrated that D2 receptor ligands can modulate DA uptake measured in rat striatum both in uiuo and in uitro (3). In situ DAT activity in the brain can be monitored using electrochemical recording( s) to measure local changes in clearance of exogenously applied DA Advances in Pharmacology, Volume 42 Copyright 0 1998 hy Academic Press. All rights ot reproduction In any form reserved 1054-328Y1Y8 $25.00 I95